It has been conventional in this kind of apparatus to arrange a plurality of sonic nozzles in a vertically disposed relationship within an intermediate portion of a single common nozzle chamber such that these nozzles may be opened and closed individually by means of respective corresponding valves. In this conventional arrangement, however, the chamber is comparatively large in size and this results in the formation of an expanded or enlarged chamber at the intermediate portion of the measuring pipe passage whereby the volume of the interior of the pipe passage is increased and this is liable to become an unnecessary load for pressure adjustment. Additionally, fuel within an air-fuel mixture is liable to be accumulated at the bottom of the chamber, and this accumulated fuel can become the cause of measurement error. Therefore, its removal is essential, but in order to do so it is necessary to stop the testing operation during such removal resulting in inefficient operation.